ψK⁎0

Physics Letters B 719 (2013) 318–325

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Physics Letters B www.elsevier.com/locate/physletb

Measurement of the B 0 –B 0 oscillation frequency md with the decays B 0 → D − π + and B 0 → J /ψ K ∗0 ✩ .LHCb Collaboration a r t i c l e

i n f o

Article history: Received 25 October 2012 Received in revised form 18 December 2012 Accepted 9 January 2013 Available online 16 January 2013 Editor: M. Doser

a b s t r a c t The B 0 –B 0 oscillation frequency md is measured by the LHCb experiment √ using a dataset corresponding to an integrated luminosity of 1.0 fb−1 of proton–proton collisions at s = 7 TeV, and is found to be md = 0.5156 ± 0.0051 (stat.) ± 0.0033 (syst.) ps−1 . The measurement is based on results from analyses of the decays B 0 → D − π + (D − → K + π − π − ) and B 0 → J /ψ K ∗0 ( J /ψ → μ+ μ− , K ∗0 → K + π − ) and their charge conjugated modes. © 2013 CERN. Published by Elsevier B.V. All rights reserved.

1. Introduction The frequency md of oscillations between B 0 mesons and B 0 mesons also describes the mass difference md between the physical eigenstates in the B 0 –B 0 system, and has been measured at LEP [1], the Tevatron [2,3], and the B factories [4,5]. The current world average is md = 0.507 ± 0.004 ps−1 [6], whilst the best single measurement prior to this Letter is by the Belle experiment, md = 0.511 ± 0.005 (stat.) ± 0.006 (syst.) ps−1 [5]. In this document the convention h¯ = c = 1 is used for all units. With increasing accuracy of the measurement of ms , the counterpart of md in the B 0s –B 0s system [7], a more precise knowledge of md becomes important, as the ratio md /ms together with input from lattice QCD calculations [8,9] constrains the apex of the CKM unitarity triangle [10,11]. Therefore, the measurement of md provides an important test of the Standard Model [12,13]. Furthermore, md is an input parameter in the determination of sin 2β at LHCb [14]. This Letter presents a measurement of m√ d , using a dataset corresponding to 1.0 fb−1 of pp collisions at s = 7 TeV, using the decay channels B 0 → D − π + (D − → K + π − π − ) and B 0 → J /ψ K ∗0 ( J /ψ → μ+ μ− , K ∗0 → K + π − ) and their charge conjugated modes. For a measurement of md , the flavour of the B 0 meson at production and decay must be known. The flavour at decay is determined in both decay channels from the charge of the final state kaon; contributions from suppressed B 0 → D + π − amplitudes are negligible. The determination of the flavour at production is achieved by the flavour tagging algorithms which are described in more detail in Section 4. The B 0 meson is defined as unmixed (mixed) if the production flavour is equal (not equal) to the flavour at decay. With this

✩

© CERN for the benefit of the LHCb Collaboration.

0370-2693/ © 2013 CERN. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physletb.2013.01.019

knowledge, the oscillation frequency md of the B 0 meson can be determined using the time dependent mixing asymmetry signal

Amix (t ) =

N unmixed (t ) − N mixed (t ) N unmixed (t ) + N mixed (t )

= cos(md t ),

(1)

where t is the B 0 decay time and N (un)mixed is the number of (un)mixed events. 2. Experimental setup and datasets The LHCb detector [15] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift-tubes placed downstream. The combined tracking system has a momentum resolution  p / p that varies from 0.4% at 5 GeV to 0.6% at 100 GeV, and an impact parameter (IP) resolution of 20 μm for tracks with high transverse momentum. Charged hadrons are identified using two ring-imaging Cherenkov detectors. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and pre-shower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers. The trigger consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage which applies a full event reconstruction. Events including B 0 → D − π + decays are required to have tracks with high transverse momentum p T to pass the hardware trigger. The software trigger requires a two-, three- or four-track secondary vertex with a large sum of the p T of the tracks, significant displacement from the associated primary vertex (PV), and at

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Fig. 1. Distribution of the B 0 candidate mass (black points). (Left) B 0 → D − π + candidates with the invariant mass PDF as described in Section 6 and two additional components for the physics background taken from MC simulated events. The blue dashed line shows the fit projection of the signal, the dotted orange line corresponds to the combinatorial background, the filled areas represent the physics background, and the black solid line corresponds to the fit projection. (Right) B 0 → J /ψ K ∗0 candidates, with the results of the fits described in Section 6 superimposed. The blue dashed line shows the fit projection of the signal, the dotted orange line corresponds to the combinatorial background with long lifetime and the dash dotted red line shows the combinatorial background with short lifetime. The black solid line corresponds to the fit projection.

least one track with p T > 1.7 GeV and a large impact parameter with respect to that PV, and a good track fit. A multivariate algorithm is used for the identification of the secondary vertices [16]. Events in the decay B 0 → J /ψ K ∗0 are first required to pass a hardware trigger which selects a single muon with p T > 1.48 GeV. In the subsequent software trigger [16], at least one of the final state particles is required to have p T > 0.8 GeV and a large IP with respect to all PVs in the event. Finally, the tracks of two or more of the final state particles are required to form a vertex which is significantly displaced from the PVs in the event. For the simulation studies, pp collisions are generated using Pythia 6.4 [17] with a specific LHCb configuration [18]. Decays of hadronic particles are described by EvtGen [19] in which final state radiation is generated using Photos [20]. The interaction of the generated particles with the detector and its response are implemented using the Geant4 toolkit [21,22] as described in Ref. [23]. 3. Selection The decay time t of a B 0 candidate is evaluated from the measured momenta and from a vertex fit that constrains the B 0 candidate to originate from the associated PV [24], and using t =  · m( B 0 )/ p, with the flight distance . The associated PV is the primary vertex that is closest to the decaying B 0 meson. No mass constraints on the intermediate resonances are applied. For the calculation of the invariant mass m, no mass constraints are used in the B 0 → D − π + channel, while the J /ψ mass is constrained to the world average [6] in the analysis of the decay B 0 → J /ψ K ∗0 . All kaons, pions and muons are required to have large p T and well reconstructed tracks and vertices. In addition to this, particle identification is used to distinguish between pion, kaon and proton tracks. The B 0 → D − π + selection requires that the D − reconstructed mass be in a range of ±100 MeV around the world average [6]. Furthermore, the D − decay vertex is required to be downstream of the PV associated to the B 0 candidate. The sum of the D − and π + p T must be larger than 5 GeV. The B 0 candidate invariant mass must be in the interval 5000  m( K + π − π − π + ) < 5700 MeV. Additionally, the cosine of the pointing angle between the B 0 momentum vector and the line segment between PV and secondary vertex is required to be larger than 0.999. Candidates are classified by a boosted decision tree (BDT) [25, 26] with the AdaBoost algorithm [27]. The BDT is trained with + candidates with no particle ID criteria applied to the B 0s → D − s π

daughter pions and kaons. The cut on the BDT classifier is optimised in order to maximise the significance of the B 0 → D − π + signal. Several input variables are used: the IP significance, the flight distance perpendicular to the beam axis, the vertex quality of the B 0 and the D − candidate, the angle between the B 0 momentum and the line segment between PV and B 0 decay vertex, the angle between the D − momentum and the line segment between PV and the D − decay vertex, the angle between the D − momentum and the line segment between the B 0 decay vertex and D − decay vertex, the IP and p T of the π + track, and the angle between the π + momentum and the line segment between PV and B 0 decay vertex. Only B 0 candidates with a decay time t > 0.3 ps are accepted. To suppress potential background from misidentified kaons in − + − decays, all D − candidates are removed if they D− s → K K π have a daughter pion candidate that might pass a loose kaon selection and are within a ±25 MeV mass window (the D − mass resolution is smaller than 10 MeV) around the D − s mass when that pion is reconstructed under the kaon mass hypothesis. Remaining background comes from B 0 → D − ρ + and B 0 → D ∗− π + decays. In both cases the final state is similar to the signal, except for an additional neutral pion that is not reconstructed. This leads to two additional peaking components with invariant masses lower than those of the signal candidates. Therefore, for the measurement of md only candidates with an invariant mass in the range 5200  m < 5450 MeV are used. The B 0 → J /ψ K ∗0 selection requires that the K ∗0 candidate has a p T > 2 GeV and 826  m( K + π − ) < 966 MeV. The unconstrained μ+ μ− invariant mass must be within ±80 MeV of the J /ψ mass [6]. B 0 candidates are required to have a large IP with respect to other PVs in the event and the B 0 decay vertex must be significantly separated from the PV. Additionally, B 0 candidates are required to have a reconstructed decay time t > 0.3 ps and an invariant mass in the range 5230  m( J /ψ K + π − ) < 5330 MeV. To suppress potential background from misidentified B 0s → J /ψφ decays, all candidates are removed for which the K + π − mass is within a ±10 MeV window around the nominal φ(1020) mass when computed under the kaon mass hypothesis for the pion. The resulting mass distributions for the two decay channels are shown in Fig. 1. 4. Flavour tagging This analysis makes use of a combination of opposite side taggers and the same side pion tagger to determine the flavour of the B 0 meson at production. The opposite side taggers, which use

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decay products of the b quark not belonging to the signal decay, are described in detail in Ref. [28]. The same side pion tagger uses the charge of a pion that originates from the fragmentation process of the B 0 meson or from decays of charged excited B mesons. Pion tagging candidates are required to fulfil criteria on p T and particle identification, as well as their IP significance and the difference between the B 0 candidate mass and the combined mass of the B 0 candidate and the pion [29]. Depending on the tagging decision, a mixing state q is assigned to each candidate, to distinguish the unmixed (q = +1) from the mixed (q = −1). Untagged events (q = 0) are not used in this analysis. The tag and its predicted wrong tag probability ηc are evaluated for each event using a neural network calibrated and optimised on B + → J /ψ K + , B 0 → J /ψ K ∗0 and B 0 → D ∗− μ+ νμ events. To take into account a possible difference in the overall tagging performance between the calibration channels and the decay channels used in this analysis, the corrected wrong tag probability ω assigned to each event is parametrised as a linear function of ηc (the method is described and tested in Ref. [28])





ω(ηc | p 0 , p 1 ) = p 0 + p 1 ηc − ηc  ,

(2)

where p 0 and p 1 are free parameters in the fit for md described in Section 6. In this way, uncertainties due to the overall calibration of the tagging performance are absorbed in the statistical uncertainty on md returned by the fit. 5. Decay time resolution and acceptance The decay time resolution of the detector is around 0.05 ps [30]. This is small compared to the B 0 oscillation period of about 12 ps and does not have significant impact on the measurement of md . The resolution is accounted for by convolving a Gaussian function G (t ; σt ), using a fixed width σt = 0.05 ps, with the signal probability density function (PDF) from Eq. (5). Possible systematic uncertainties introduced by the resolution are discussed in Section 7. Trigger, reconstruction and selection criteria introduce efficiency effects that depend on the decay time. While these effects cancel in the asymmetry of Eq. (1) for signal events, they can be important for event samples that include background. As will be shown in Section 6, the only relevant background in the B 0 signal region is combinatorial in nature. For this background the asymbkg

bkg

metry N q=1 (t ) − N q=−1 (t ) is expected to cancel to first order as q has no physical meaning. Therefore,

Amix (t ) ∝

∝

sig

bkg

sig

bkg

sig

bkg

sig

bkg

( N q=1 (t ) + N q=1 (t )) − ( N q=−1 (t ) + N q=−1 (t )) ( N q=1 (t ) + N q=1 (t )) + ( N q=−1 (t ) + N q=−1 (t )) S (t ) S (t ) + B (t )

sig,bkg

(3)

cos(md t ),

where N q=±1 (t ) denotes the number of unmixed or mixed signal (sig) and background (bkg) events. S (t ) and B (t ) denote the number of signal and background events as a function of the decay time. Thus, the shapes of S (t ) and B (t ) have to be known to account for the time dependent amplitude of the asymmetry function. In the analysis of decays B 0 → J /ψ K ∗0 , the decay time acceptance is determined from data, using a control sample of B 0 → J /ψ K ∗0 events that is collected without applying any of the decay time biasing selection criteria. The decay time acceptance is evaluated in bins of t and is implemented in the fit described in Section 6.

In the decay B 0 → D − π + there is no control dataset that can be used to measure the decay time acceptance. From an analysis of simulated events, it is determined that the decay time acceptance can be described by the empirical function





acc (t |a1 , a2 ) = arctan a1 exp(a2t ) ,

(4)

where the parameters a1 and a2 are both free in the maximum likelihood fit for md described in Section 6. 6. Measurement of md The value of md is measured using a multi-dimensional extended maximum likelihood fit. The B 0 → D − π + data are described by a two component PDF in which one component describes the signal and the other describes the combinatorial background. The signal component consists of the sum of a Gaussian function and a Crystal Ball function [31] with a common mean t for the mass distribution, multiplied by a function Psig to describe the decay time distribution, t Psig (t , q; τ , md , ω, σt , a1 , a2 )     t  ∝ Θ(t − 0.3 ps) · e− τ 1 + q 1 − 2ω(ηc | p 0 , p 1 ) cos(md t )  (5) ⊗ G (t ; σt ) · acc (t |a1 , a2 ).

Here, Θ(t ) is the step function, while the B 0 lifetime τ is a free fit parameter and the average decay time resolution σt is fixed. Other fit parameters are a1 and a2 from the decay time acceptance function acc (t |a1 , a2 ) described in Section 5, as well as the parameters p 0 and p 1 from the tagging calibration function ω(ηc | p 0 , p 1 ) described in Section 4. Any B 0 / B 0 production asymmetry cancels in the mixing asymmetry function, and is neglected in this analysis. The combinatorial background component consists of an exponential PDF describing the mass distribution and the decay time PDF t Pbkg (t , q; τbkg , ωbkg , σt )

   − t  ∝ Θ(t − 0.3 ps) · e τbkg 1 + q(1 − 2ωbkg ) ⊗ G (t ; σt ) .

(6)

The PDF is similar to the signal decay time PDF with md fixed to zero. The parameter ωbkg allows the PDF to reflect a possible asymmetry in the number of events tagged with q = ±1 in the background. The effective lifetime τbkg of the long-lived background component is allowed to vary independently in the fit. Possible backgrounds from misidentified or partially reconstructed decays are studied using mass templates determined from simulation. These are found to be negligible in the mass window 5200  m( K + π − π − π + ) < 5450 MeV that is used in the fit (cf. Fig. 1). In the B 0 → J /ψ K ∗0 analysis, the signal mass distribution is modelled by a double Gaussian function with a common mean and the decay time PDF is the same as described in Eq. (5), except for the decay time acceptance acc (t |a1 , a2 ) that is replaced by the acceptance histogram described in Section 5 and has no free parameters. The mass distribution of the combinatorial background in B 0 → J /ψ K ∗0 decays is also described by an exponential function. However, the decay time distribution includes a second component of shorter lifetime to account for prompt J /ψ candidates passing the selection. The long-lived component is described by the same function as the combinatorial background in B 0 → D − π + decays as in Eq. (6), whereas the short-lived component is described by a simple exponential function. No other significant source of background is found.

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Fig. 2. Distribution of the decay time (black points) for (left) B 0 → D − π + and (right) B 0 → J /ψ K ∗0 candidates. The blue dashed line shows the fit projection of the signal, the dotted orange line corresponds to the combinatorial background with long lifetime and the dash dotted red line shows the combinatorial background with short lifetime (only in the B 0 → J /ψ K ∗0 mode). The black solid line corresponds to the projection of the combined PDF.

Fig. 3. Raw mixing asymmetry Amix (black points) for (left) B 0 → D − π + and (right) B 0 → J /ψ K ∗0 candidates. The solid black line is the projection of the mixing asymmetry of the combined PDF.

The resulting values for md are 0.5178 ± 0.0061 ps−1 and 0.5096 ± 0.0114 ps−1 in the B 0 → D − π + and B 0 → J /ψ K ∗0 decay modes respectively. The fit yields 87 724 ± 321 signal decays for B 0 → D − π + and 39 148 ± 316 signal decays for B 0 → J /ψ K ∗0 . The fit projections onto the decay time distributions are displayed in Fig. 2 and the resulting asymmetries are shown in Fig. 3. No result for the B 0 lifetime is quoted, since it is affected by possible biases due to acceptance corrections. These acceptance effects do not influence the measurement of md . 7. Systematic uncertainties As explained in Section 5, systematic effects due to the decay time resolution are expected to be small. This is tested using samples of simulated events that are generated with decay time distributions given by the result of the fit to data and convolved with the average measured decay time resolution of 0.05 ps. The event samples are then fitted with the PDF described in Section 6, with the decay time resolution parameter fixed either to zero or to σt = 0.10 ps. The maximum observed bias on md of 0.0002 ps−1 is assigned as systematic uncertainty. Systematic effects due to decay time acceptance are estimated in a similar study, generating samples of simulated events according to the nominal decay time acceptance functions described in Section 5. These samples are then fitted with the PDF described in Section 6, but neglecting the decay time acceptance function in the fit. The average observed shift of 0.0004 ps−1 (0.0001 ps−1 ) in B 0 → D − π + (B 0 → J /ψ K ∗0 ) decays is taken as systematic uncertainty. The influence of eventby-event variation of the decay time resolution is found to be negligible.

In order to estimate systematic effects due to the parametrisation of the decay time PDFs for signal and background, an alternative parametrisation is derived with a data-driven method, using sWeights [32] from a fit to the mass distribution. The sWeighted decay time distributions for the signal and background components are then described by Gaussian kernel PDFs, which replace the exponential terms of the decay time PDF. This leads to a description of the data which is independent of a model for the decay time and its acceptance, that can be used to fit for md . The resulting shifts of 0.0037 ps−1 (0.0022 ps−1 ) in the decay B 0 → D − π + (B 0 → J /ψ K ∗0 ) are taken as the systematic uncertainty due to the fit model. Uncertainties in the geometric description of the detector lead to uncertainties in the measurement of flight distances and the momenta of final state particles. From alignment measurements on the vertex detector, the relative uncertainty on the length scale is known to be smaller than 0.1%. This uncertainty translates directly into a relative systematic uncertainty on md , yielding an absolute uncertainty of 0.0005 ps−1 . From measurements of biases in the reconstructed J /ψ mass in several run periods, the relative uncertainty on the uncalibrated momentum scale is measured to be smaller than 0.15%. This uncertainty, however, cancels to a large extent in the calculation of the B 0 decay time, as it affects both the reconstructed B 0 momentum and its reconstructed mass, which is dominated by the measured momenta of the final state particles. The remaining systematic uncertainty on the decay time is found to be an order of magnitude smaller than that due to the length scale and is neglected. A summary of the systematic uncertainties can be found in Table 1. The systematic uncertainty on the combined md result is calculated using a weighted average of the combined uncorrelated

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B 0 → J /ψ K ∗0

B0 → D−π +

tion License 3.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are credited.

Acceptance Decay time resolution Fit model

0.0001 0.0002 0.0022

0.0004 0.0002 0.0037

References

Total uncorrelated

0.0022

0.0037

Length scale

0.0005

0.0005

Total including correlated

0.0023

0.0037

Table 1 Systematic uncertainties on md in ps−1 .

uncertainties in both channels. The uncertainty on the length scale is fully correlated across the channels and therefore added after the combination. 8. Conclusion The B 0 –B 0 oscillation frequency md has been measured using samples of B 0 → D − π + √ and B 0 → J /ψ K ∗0 events collected in 1.0 fb−1 of pp collisions at s = 7 TeV and is found to be

  md B 0 → D − π + = 0.5178 ± 0.0061 (stat.) 

md B 0 → J /ψ K

 ∗0

± 0.0037 (syst.) ps−1 and = 0.5096 ± 0.0114 (stat.) ± 0.0022 (syst.) ps−1 .

The combined value for md is calculated as the weighted average of the individual results taking correlated systematic uncertainties into account

md = 0.5156 ± 0.0051 (stat.) ± 0.0033 (syst.) ps−1 . It is currently the most precise measurement of this parameter. The relative uncertainty on md is 1.2%, where it is around 0.6% for ms [7]. Thus, the uncertainty on the ratio md /ms is dominated by md . As the systematic uncertainties in the md and ms measurements are small, the error on the ratio can be further improved with more data. Acknowledgements We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 and Region Auvergne (France); BMBF, DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO (The Netherlands); SCSR (Poland); ANCS/IFA (Romania); MinES, Rosatom, RFBR and NRC “Kurchatov Institute” (Russia); MinECo, XuntaGal and GENCAT (Spain); SNSF and SER (Switzerland); NAS Ukraine (Ukraine); STFC (United Kingdom); NSF (USA). We also acknowledge the support received from the ERC under FP7. The Tier1 computing centres are supported by IN2P3 (France), KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands), PIC (Spain), GridPP (United Kingdom). We are thankful for the computing resources put at our disposal by Yandex LLC (Russia), as well as to the communities behind the multiple open source software packages that we depend on. Open access This article is published Open Access at sciencedirect.com. It is distributed under the terms of the Creative Commons Attribu-

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R. Aaij 38 , C. Abellan Beteta 33,n , A. Adametz 11 , B. Adeva 34 , M. Adinolfi 43 , C. Adrover 6 , A. Affolder 49 , Z. Ajaltouni 5 , J. Albrecht 35 , F. Alessio 35 , M. Alexander 48 , S. Ali 38 , G. Alkhazov 27 , P. Alvarez Cartelle 34 , A.A. Alves Jr. 22 , S. Amato 2 , Y. Amhis 36 , L. Anderlini 17,f , J. Anderson 37 , R.B. Appleby 51 , O. Aquines Gutierrez 10 , F. Archilli 18,35 , A. Artamonov 32 , M. Artuso 53 , E. Aslanides 6 , G. Auriemma 22,m , S. Bachmann 11 , J.J. Back 45 , C. Baesso 54 , W. Baldini 16 , R.J. Barlow 51 , C. Barschel 35 , S. Barsuk 7 , W. Barter 44 , A. Bates 48 , Th. Bauer 38 , A. Bay 36 , J. Beddow 48 , I. Bediaga 1 , S. Belogurov 28 , K. Belous 32 , I. Belyaev 28 , E. Ben-Haim 8 , M. Benayoun 8 , G. Bencivenni 18 , S. Benson 47 , J. Benton 43 , A. Berezhnoy 29 , R. Bernet 37 , M.-O. Bettler 44 , M. van Beuzekom 38 , A. Bien 11 , S. Bifani 12 , T. Bird 51 , A. Bizzeti 17,h , P.M. Bjørnstad 51 , T. Blake 35 , F. Blanc 36 , C. Blanks 50 , J. Blouw 11 , S. Blusk 53 , A. Bobrov 31 , V. Bocci 22 , A. Bondar 31 , N. Bondar 27 , W. Bonivento 15 , S. Borghi 48,51 , A. Borgia 53 , T.J.V. Bowcock 49 , C. Bozzi 16 , T. Brambach 9,∗ , J. van den Brand 39 , J. Bressieux 36 , D. Brett 51 , M. Britsch 10 , T. Britton 53 , N.H. Brook 43 , H. Brown 49 , A. Büchler-Germann 37 , I. Burducea 26 , A. Bursche 37 , J. Buytaert 35 , S. Cadeddu 15 , O. Callot 7 , M. Calvi 20,j , M. Calvo Gomez 33,n , A. Camboni 33 , P. Campana 18,35 , A. Carbone 14,c , G. Carboni 21,k , R. Cardinale 19,i , A. Cardini 15 , H. Carranza-Mejia 47 , L. Carson 50 , K. Carvalho Akiba 2 , G. Casse 49 , M. Cattaneo 35 , Ch. Cauet 9 , M. Charles 52 , Ph. Charpentier 35 , P. Chen 3,36 , N. Chiapolini 37 , M. Chrzaszcz 23 , K. Ciba 35 , X. Cid Vidal 34 , G. Ciezarek 50 , P.E.L. Clarke 47 , M. Clemencic 35 , H.V. Cliff 44 , J. Closier 35 , C. Coca 26 , V. Coco 38 , J. Cogan 6 , E. Cogneras 5 , P. Collins 35 , A. Comerma-Montells 33 , A. Contu 52,15 , A. Cook 43 , M. Coombes 43 , G. Corti 35 , B. Couturier 35 , G.A. Cowan 36 , D. Craik 45 , S. Cunliffe 50 , R. Currie 47 , C. D’Ambrosio 35 , P. David 8 , P.N.Y. David 38 , I. De Bonis 4 , K. De Bruyn 38 , S. De Capua 51 , M. De Cian 37 , J.M. De Miranda 1 , L. De Paula 2 , P. De Simone 18 , D. Decamp 4 , M. Deckenhoff 9 , H. Degaudenzi 36,35 , L. Del Buono 8 , C. Deplano 15 , D. Derkach 14 , O. Deschamps 5 , F. Dettori 39 , A. Di Canto 11 , J. Dickens 44 , H. Dijkstra 35 , P. Diniz Batista 1 , M. Dogaru 26 , F. Domingo Bonal 33,n , S. Donleavy 49 , F. Dordei 11 , A. Dosil Suárez 34 , D. Dossett 45 , A. Dovbnya 40 , F. Dupertuis 36 , R. Dzhelyadin 32 , A. Dziurda 23 , A. Dzyuba 27 , S. Easo 46,35 , U. Egede 50 , V. Egorychev 28 , S. Eidelman 31 , D. van Eijk 38 , S. Eisenhardt 47 , R. Ekelhof 9 , L. Eklund 48 , I. El Rifai 5 , Ch. Elsasser 37 , D. Elsby 42 , A. Falabella 14,e , C. Färber 11 , G. Fardell 47 , C. Farinelli 38 , S. Farry 12 , V. Fave 36 , V. Fernandez Albor 34 , F. Ferreira Rodrigues 1 , M. Ferro-Luzzi 35 , S. Filippov 30 , C. Fitzpatrick 35 , M. Fontana 10 , F. Fontanelli 19,i , R. Forty 35 , O. Francisco 2 , M. Frank 35 , C. Frei 35 , M. Frosini 17,f , S. Furcas 20 , A. Gallas Torreira 34 , D. Galli 14,c , M. Gandelman 2 , P. Gandini 52 , Y. Gao 3 , J.-C. Garnier 35 , J. Garofoli 53 , P. Garosi 51 , J. Garra Tico 44 , L. Garrido 33 , C. Gaspar 35 , R. Gauld 52 , E. Gersabeck 11 , M. Gersabeck 35 , T. Gershon 45,35 , Ph. Ghez 4 , V. Gibson 44 , V.V. Gligorov 35 , C. Göbel 54 , D. Golubkov 28 , A. Golutvin 50,28,35 , A. Gomes 2 , H. Gordon 52 , M. Grabalosa Gándara 33 , R. Graciani Diaz 33 , L.A. Granado Cardoso 35 , E. Graugés 33 , G. Graziani 17 , A. Grecu 26 , E. Greening 52 , S. Gregson 44 , O. Grünberg 55 , B. Gui 53 , E. Gushchin 30 , Yu. Guz 32 , T. Gys 35 , C. Hadjivasiliou 53 , G. Haefeli 36 , C. Haen 35 , S.C. Haines 44 , S. Hall 50 , T. Hampson 43 , S. Hansmann-Menzemer 11 , N. Harnew 52 , S.T. Harnew 43 , J. Harrison 51 , P.F. Harrison 45 , T. Hartmann 55 , J. He 7 , V. Heijne 38 , K. Hennessy 49 , P. Henrard 5 , J.A. Hernando Morata 34 , E. van Herwijnen 35 , E. Hicks 49 , D. Hill 52 , M. Hoballah 5 , P. Hopchev 4 , W. Hulsbergen 38 , P. Hunt 52 , T. Huse 49 , N. Hussain 52 , D. Hutchcroft 49 , D. Hynds 48 , V. Iakovenko 41 , P. Ilten 12 , J. Imong 43 , R. Jacobsson 35 , A. Jaeger 11 , M. Jahjah Hussein 5 , E. Jans 38 , F. Jansen 38 , P. Jaton 36 , B. Jean-Marie 7 , F. Jing 3 , M. John 52 , D. Johnson 52 , C.R. Jones 44 , B. Jost 35 , M. Kaballo 9 , S. Kandybei 40 , M. Karacson 35 , T.M. Karbach 35 , I.R. Kenyon 42 , U. Kerzel 35 , T. Ketel 39 , A. Keune 36 , B. Khanji 20 , Y.M. Kim 47 , O. Kochebina 7 , V. Komarov 36,29 , R.F. Koopman 39 , P. Koppenburg 38 , M. Korolev 29 , A. Kozlinskiy 38 , L. Kravchuk 30 , K. Kreplin 11 , M. Kreps 45 , G. Krocker 11 , P. Krokovny 31 , F. Kruse 9 , M. Kucharczyk 20,23,j , V. Kudryavtsev 31 , T. Kvaratskheliya 28,35 , V.N. La Thi 36 , D. Lacarrere 35 , G. Lafferty 51 , A. Lai 15 , D. Lambert 47 , R.W. Lambert 39 , E. Lanciotti 35 , G. Lanfranchi 18,35 , C. Langenbruch 35 , T. Latham 45 , C. Lazzeroni 42 , R. Le Gac 6 , J. van Leerdam 38 , J.-P. Lees 4 , R. Lefèvre 5 , A. Leflat 29,35 , J. Lefrançois 7 , O. Leroy 6 , T. Lesiak 23 , Y. Li 3 , L. Li Gioi 5 , M. Liles 49 , R. Lindner 35 , C. Linn 11 , B. Liu 3 , G. Liu 35 , J. von Loeben 20 , J.H. Lopes 2 , E. Lopez Asamar 33 , N. Lopez-March 36 , H. Lu 3 , J. Luisier 36 , H. Luo 47 , A. Mac Raighne 48 , F. Machefert 7 , I.V. Machikhiliyan 4,28 , F. Maciuc 26 , O. Maev 27,35 , J. Magnin 1 , M. Maino 20 , S. Malde 52 , G. Manca 15,d , G. Mancinelli 6 , N. Mangiafave 44 , U. Marconi 14 , R. Märki 36 , J. Marks 11 , G. Martellotti 22 , A. Martens 8 , L. Martin 52 , A. Martín Sánchez 7 ,

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M. Martinelli 38 , D. Martinez Santos 35 , D. Martins Tostes 2 , A. Massafferri 1 , R. Matev 35 , Z. Mathe 35 , C. Matteuzzi 20 , M. Matveev 27 , E. Maurice 6 , A. Mazurov 16,30,35,e , J. McCarthy 42 , G. McGregor 51 , R. McNulty 12 , M. Meissner 11 , M. Merk 38 , J. Merkel 9 , D.A. Milanes 13 , M.-N. Minard 4 , J. Molina Rodriguez 54 , S. Monteil 5 , D. Moran 51 , P. Morawski 23 , R. Mountain 53 , I. Mous 38 , F. Muheim 47 , K. Müller 37 , R. Muresan 26 , B. Muryn 24 , B. Muster 36 , J. Mylroie-Smith 49 , P. Naik 43 , T. Nakada 36 , R. Nandakumar 46 , I. Nasteva 1 , M. Needham 47 , N. Neufeld 35 , A.D. Nguyen 36 , T.D. Nguyen 36 , C. Nguyen-Mau 36,o , M. Nicol 7 , V. Niess 5 , N. Nikitin 29 , T. Nikodem 11 , A. Nomerotski 52,35 , A. Novoselov 32 , A. Oblakowska-Mucha 24 , V. Obraztsov 32 , S. Oggero 38 , S. Ogilvy 48 , O. Okhrimenko 41 , R. Oldeman 15,35,d , M. Orlandea 26 , J.M. Otalora Goicochea 2 , P. Owen 50 , B.K. Pal 53 , A. Palano 13,b , M. Palutan 18 , J. Panman 35 , A. Papanestis 46 , M. Pappagallo 48 , C. Parkes 51 , C.J. Parkinson 50 , G. Passaleva 17 , G.D. Patel 49 , M. Patel 50 , G.N. Patrick 46 , C. Patrignani 19,i , C. Pavel-Nicorescu 26 , A. Pazos Alvarez 34 , A. Pellegrino 38 , G. Penso 22,l , M. Pepe Altarelli 35 , S. Perazzini 14,c , D.L. Perego 20,j , E. Perez Trigo 34 , A. Pérez-Calero Yzquierdo 33 , P. Perret 5 , M. Perrin-Terrin 6 , G. Pessina 20 , K. Petridis 50 , A. Petrolini 19,i , A. Phan 53 , E. Picatoste Olloqui 33 , B. Pie Valls 33 , B. Pietrzyk 4 , T. Pilaˇr 45 , D. Pinci 22 , S. Playfer 47 , M. Plo Casasus 34 , F. Polci 8 , G. Polok 23 , A. Poluektov 45,31 , E. Polycarpo 2 , D. Popov 10 , B. Popovici 26 , C. Potterat 33 , A. Powell 52 , J. Prisciandaro 36 , V. Pugatch 41 , A. Puig Navarro 36 , W. Qian 4 , J.H. Rademacker 43 , B. Rakotomiaramanana 36 , M.S. Rangel 2 , I. Raniuk 40 , N. Rauschmayr 35 , G. Raven 39 , S. Redford 52 , M.M. Reid 45 , A.C. dos Reis 1 , S. Ricciardi 46 , A. Richards 50 , K. Rinnert 49 , V. Rives Molina 33 , D.A. Roa Romero 5 , P. Robbe 7 , E. Rodrigues 48,51 , P. Rodriguez Perez 34 , G.J. Rogers 44 , S. Roiser 35 , V. Romanovsky 32 , A. Romero Vidal 34 , J. Rouvinet 36 , T. Ruf 35 , H. Ruiz 33 , G. Sabatino 22,k , J.J. Saborido Silva 34 , N. Sagidova 27 , P. Sail 48 , B. Saitta 15,d , C. Salzmann 37 , B. Sanmartin Sedes 34 , M. Sannino 19,i , R. Santacesaria 22 , C. Santamarina Rios 34 , R. Santinelli 35 , E. Santovetti 21,k , M. Sapunov 6 , A. Sarti 18,l , C. Satriano 22,m , A. Satta 21 , M. Savrie 16,e , P. Schaack 50 , M. Schiller 39 , H. Schindler 35 , S. Schleich 9 , M. Schlupp 9 , M. Schmelling 10 , B. Schmidt 35 , O. Schneider 36 , A. Schopper 35 , M.-H. Schune 7 , R. Schwemmer 35 , B. Sciascia 18 , A. Sciubba 18,l , M. Seco 34 , A. Semennikov 28 , K. Senderowska 24 , I. Sepp 50 , N. Serra 37 , J. Serrano 6 , P. Seyfert 11 , M. Shapkin 32 , I. Shapoval 40,35 , P. Shatalov 28 , Y. Shcheglov 27 , T. Shears 49,35 , L. Shekhtman 31 , O. Shevchenko 40 , V. Shevchenko 28 , A. Shires 50 , R. Silva Coutinho 45 , T. Skwarnicki 53 , N.A. Smith 49 , E. Smith 52,46 , M. Smith 51 , K. Sobczak 5 , F.J.P. Soler 48 , F. Soomro 18,35 , D. Souza 43 , B. Souza De Paula 2 , B. Spaan 9 , A. Sparkes 47 , P. Spradlin 48 , F. Stagni 35 , S. Stahl 11 , O. Steinkamp 37 , S. Stoica 26 , S. Stone 53 , B. Storaci 38 , M. Straticiuc 26 , U. Straumann 37 , V.K. Subbiah 35 , S. Swientek 9 , M. Szczekowski 25 , P. Szczypka 36,35 , T. Szumlak 24 , S. T’Jampens 4 , M. Teklishyn 7 , E. Teodorescu 26 , F. Teubert 35 , C. Thomas 52 , E. Thomas 35 , J. van Tilburg 11 , V. Tisserand 4 , M. Tobin 37 , S. Tolk 39 , D. Tonelli 35 , S. Topp-Joergensen 52 , N. Torr 52 , E. Tournefier 4,50 , S. Tourneur 36 , M.T. Tran 36 , A. Tsaregorodtsev 6 , P. Tsopelas 38 , N. Tuning 38 , M. Ubeda Garcia 35 , A. Ukleja 25 , D. Urner 51 , U. Uwer 11 , V. Vagnoni 14 , G. Valenti 14 , R. Vazquez Gomez 33 , P. Vazquez Regueiro 34 , S. Vecchi 16 , J.J. Velthuis 43 , M. Veltri 17,g , G. Veneziano 36 , M. Vesterinen 35 , B. Viaud 7 , I. Videau 7 , D. Vieira 2 , X. Vilasis-Cardona 33,n , J. Visniakov 34 , A. Vollhardt 37 , D. Volyanskyy 10 , D. Voong 43 , A. Vorobyev 27 , V. Vorobyev 31 , C. Voß 55 , H. Voss 10 , R. Waldi 55 , R. Wallace 12 , S. Wandernoth 11 , J. Wang 53 , D.R. Ward 44 , N.K. Watson 42 , A.D. Webber 51 , D. Websdale 50 , M. Whitehead 45 , J. Wicht 35 , D. Wiedner 11 , L. Wiggers 38 , G. Wilkinson 52 , M.P. Williams 45,46 , M. Williams 50,p , F.F. Wilson 46 , J. Wishahi 9 , M. Witek 23 , W. Witzeling 35 , S.A. Wotton 44 , S. Wright 44 , S. Wu 3 , K. Wyllie 35 , Y. Xie 47,35 , F. Xing 52 , Z. Xing 53 , Z. Yang 3 , R. Young 47 , X. Yuan 3 , O. Yushchenko 32 , M. Zangoli 14 , M. Zavertyaev 10,a , F. Zhang 3 , L. Zhang 53 , W.C. Zhang 12 , Y. Zhang 3 , A. Zhelezov 11 , L. Zhong 3 , A. Zvyagin 35 1

Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3 Center for High Energy Physics, Tsinghua University, Beijing, China 4 LAPP, Université de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France 5 Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6 CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France 7 LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France 8 LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France 9 Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany 10 Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany 11 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 12 School of Physics, University College Dublin, Dublin, Ireland 2

LHCb Collaboration / Physics Letters B 719 (2013) 318–325 13

Sezione INFN di Bari, Bari, Italy Sezione INFN di Bologna, Bologna, Italy Sezione INFN di Cagliari, Cagliari, Italy Sezione INFN di Ferrara, Ferrara, Italy Sezione INFN di Firenze, Firenze, Italy Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy Sezione INFN di Genova, Genova, Italy Sezione INFN di Milano Bicocca, Milano, Italy Sezione INFN di Roma Tor Vergata, Roma, Italy Sezione INFN di Roma La Sapienza, Roma, Italy Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland AGH University of Science and Technology, Kraków, Poland National Center for Nuclear Research (NCBJ), Warsaw, Poland Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia Institute for High Energy Physics (IHEP), Protvino, Russia Universitat de Barcelona, Barcelona, Spain Universidad de Santiago de Compostela, Santiago de Compostela, Spain European Organization for Nuclear Research (CERN), Geneva, Switzerland Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland Physik-Institut, Universität Zürich, Zürich, Switzerland Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine University of Birmingham, Birmingham, United Kingdom H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom Department of Physics, University of Warwick, Coventry, United Kingdom STFC Rutherford Appleton Laboratory, Didcot, United Kingdom School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom Imperial College London, London, United Kingdom School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom Department of Physics, University of Oxford, Oxford, United Kingdom Syracuse University, Syracuse, NY, United States Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil q Institut für Physik, Universität Rostock, Rostock, Germany r

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

* a b c d e f

Corresponding author. E-mail address: [email protected] (T. Brambach). P.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia. Università di Bari, Bari, Italy. Università di Bologna, Bologna, Italy. Università di Cagliari, Cagliari, Italy. Università di Ferrara, Ferrara, Italy.

g

Università di Firenze, Firenze, Italy. Università di Urbino, Urbino, Italy.

h

Università di Modena e Reggio Emilia, Modena, Italy.

i

Università di Genova, Genova, Italy.

j

Università di Milano Bicocca, Milano, Italy.

k

Università di Roma Tor Vergata, Roma, Italy.

l

Università di Roma La Sapienza, Roma, Italy. Università della Basilicata, Potenza, Italy. LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain. Hanoi University of Science, Hanoi, Viet Nam. Massachusetts Institute of Technology, Cambridge, MA, United States. Associated to: Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil. Associated to: Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany.

m n o p q r

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